[scripturial]

If you think about it, every function already declares what it needs simply by actually using it. You know if a function needs another function because it calls it. So what exactly are you asking? That the programmer insert a list of dependent functions in a comment above every function? The compiler could do that for you. The compiler could help you and go up a level and insert the names of modules the functions belong to?

[zahlman]

My understanding is that the existing algorithms for tree shaking (dead code elimination, etc. etc. whatever you want to call it) work exactly on that basis. But Python is too dynamic to just read the source code and determine what's used ahead of time. eval and exec exist; just about every kind of namespace is reflected as either a dictionary or an object with attributes, and most are mutable; and the import system works purely at runtime and has a dazzling array of hooks.